Cardiac arrhythmias are frequent in the elderly population, perhaps secondary to an increased prevalence of hypertension and coronary artery disease as well as aging related changes resulting in loss of pacemaker cells and degenerative alteration of the conduction system. Independent from underlying structural heart disease, advanced age alone appears to be a risk factor for increased susceptibility to ventricular arrhythmia. However, the electrophysiological basis of this phenomenon is still unclear. Thus, it is important to assess and to define the underlying arrhythmogenic substrate. The aim of the present study was to identify a likely structural-functional ventricular arrhythmogenic substrate in aged hearts. For this purpose ventricular activation patterns were measured in control (n=4) and aged (n=10) in vivo rat hearts by recording unipolar electrograms with an epicardial, 1 mm resolution, 8x8 electrode array, during pacing and spontaneous or induced ventricular ectopic beats. Our results in aged hearts suggest that peripheral conduction system might be involved in perpetuating sequences of ventricular ectopic beats, regardless of their origin.